Helical resonator with variable capacitor having fixed plate which also functions as inductance



April 1966 R. c. BUETOW ETAL 3,

HELICAL RESONATOR WITH VARIABLE CAPACITOR HAVING FIXED PLATE WHICH ALSO FUNCTIONS AS INDUCTANCE Filed Sept. 6, 1963 INVENTORS Richard C. Buefow BY Dale L. .Shu/f United States Patent 3,247,475 HELICAL RESONATOR WITH VARIABLE CAPACI- TOR HAVING FIXED PLATE WHICH ALSO FUNCTIONS AS INDUCTAN CE Richard C. Buetow, Mount Prospect, and Dale L. Shult, Niles, Ill., assignors to Motorola, Inc., Franklin Park, 11]., a corporation of Illinois Filed Sept. 6, 1963, Ser. No. 307,167 4 Claims. (Cl. 333-83) This invention relates to electrical apparatus responsive to applied signals of high frequency, and more particularly to an improved construction for a helical resonator.

It has been proposed to use helical resonators as tuning elements in high frequency circuits. Basically, a helical resonator is an inductance coil contained within a cavity such that the combination is resonant at a predetermined frequency. Because the physical dimensions of the components of the resonator directly determine the resonant frequency, it is essential to hold close tolerances in coil and cavity and this results in relatively high manufacturing costs. To reduce these costs, helical resonators have been constructed to be resonant at a higher frequency than is desired, and then end loaded with an adjustable capacitor to tune the resonator to a lower frequency.

A major application for helical resonators is as radio frequency (RF) coupling circuits. A helical resonator can be used, for example, in the coupling circuit between the antenna and first radio frequency amplifier stage of a radio receiver, or between subsequent RF stages of the receiver. It may be desired to use a plurality of helical resonators in a single RF coupling circuit, with the energy passing through successive ones of the resonators.

As mentioned before, end loading a helical resonator permits a variation in the resonant frequency thereof. This variation however, may not be Wide enough to allow the same basic unit to be adjusted for operation over a sufficient range of frequencies to permit mass production of the components. For example, where a large number of receivers which operate at different specified frequencies are manufactured, the variable end loading may not be adequate to vary the frequency of a resonator to all of the desired specified frequencies so that the same resonator construction might be used in all receivers. Such circumstances, of course, mean that mass production of the helical resonators is not possible since the size and dimension of each resonator must be different in order to achieve the desired resonant frequencies over the full range of frequencies being used.

Accordingly, it is an object of this invention to provide an improved construction for a helical resonator which permits the basic components thereof to be mass produced, While still providing for suflicient variation of the resonant frequency thereof as to cover a relatively wide frequency range.

It is another object of this invention to provide a helical resonator of simple, rugged construction wherein the inductance of the coil, and the capacity of the end loading of the coil may be varied.

A feature of the invention is the provision, in a helical resonator, of a coil and a flat tab which may be connected to an end thereof to add to or subtract from the inductance of the coil, and at the same time function as one plate of an adjustable end loading capacitor.

Another feature of the invention is the provision, in a 3,247,475 Patented Apr. 19, 1966 ice helical resonator, of a coil including an end portion comprising a flattened end loading capacitor tab extending in the same direction as the rest of the coil to comprise a part of the inductance thereof.

Still another feature of the invention is the provision, ina helical resonator, of a coil with a flattened end loading capacitor tab extending in a direction opposite to the rest of the coil to reduce the over-all inductance thereof.

Referring now to the drawing:

FIG. 1 is a half-sectional view of a receiver RF coupling circuit utilizing helical resonators constructed in accordance with the invention;

FIG. 2 is an exploded view of an inductance coil and end loading capacitor for a helical resonator constructed in accordance with the invention;

FIG. 3 is a perspective view of an alternative construc- 'tion for the coil; and

FIG. 4 is a perspective view of a further alternative construction for the coil. 7

In accordance with the invention, a helical resonator includes a conductive housing forming a cavity and an inductance member disposed in the cavity. The inductance member includes a helix and a fiat tab member extending therefrom. An annular member is disposed in the cavity and is adjustable therein. The annular member has a surface adjacent the tab member to form a capacitor therewith. Variations of frequency of the resonator over a wide range are attained by altering the size, configuration, and direction of the tab member. Since the tab member comprises a part of both the end loading capacitor and the inductance, numerous variations may be made in the inductance and capacitance of the different resonators to achieve a specific-ally desired frequency. Further, the capacitor formed is adjustable and end loads the coil such that the over-all capacity of the helical resonator may be varied. The resonant frequency thereof is varied accordingly, Within a given range, by movement of the adjustable annular member.

Referring now to FIG. 1 there is depicted an RF coupling circuit constructed in accordance with the invention. The RF coupling circuit shown as a particular form of the invention is for use between an antenna and the first RF amplifier. stage in the receiver. The coupling circuit includes a conductive housing for a plurality of helical resonators, and comprises a lower portion 11 and a top plate 13. Lower portion 11 includes a plurality of dividing walls or partitions 12 which, with top plate 13, form a plurality of cavities 15. Apertures 17 are provided in the partitions 12 between cavities 15 to transfer energy therebetween. The input from the antenna is applied to the coil in the furthest left of cavities 15. Energy transfer takes place between the respective cavities through apertures 17, and is taken off the coil in the furthest right hand cavity and applied to the radio frequency amplifier.

Referring to FIG. 2, the inner elements of the helical resonator include a ceramic core 19 which is fastened to the bottom of lower portion 11 by a screw 21 and Washer 23 in order to maintain mechanical rigidity'so that the resonator is protected from excessive vibration. Ideally, an air core should be used, but structural support problems dictate that the ceramic core be used.

An inductance coil 25 is disposed on core 19 and may be of silver plated copper wire. The number of turns of coil 25 depends, of course, on the frequency being utilized. However, as will be explained, the coils for all the resonators may be made identical for a certain segment of frequencies within a frequency range, due to the variability permitted by the end loading capacitor tab.

Inductance coil 25 is end loaded by a capacitor. This capacitor is comprised of a flat conductive tab 27 extending from one end of coil 25, and an annular adjustable member 29. Tab 27 is supported by the core 19 as shown in FIG. 2 and is disposed in a direction opposite to the winding of coil 25. Accordingly, the tab 27 is inductively subtractive to the coil 25. As an alternative, the tab may be disposed as shown by tab 31 in FIG. 3. In FIG. 3, tab 31' is inductively additive to the coil 25. Thus by using tabs disposed in different directions, the effective inductance of the coil can be increased or decreased. By changing the length thickness and Width of the tabs, the inductance as well as the range of variability of capacitance, can be controlled.

Annular element 29 is adjustable by means of a threaded shaft 33 which extends through a tapped opening in top plate 13. As may be seen from FIG. 2, annular member 29'includes a split 35. This is placed in annular member 29 to prevent the same from becoming a closed loop or shorted turn in the inductance of the helical resonator.

The invention provides a very large cost saving when devices of this type are manufactured in large quantities. For example, a segment of the frequency range may be selected, such as 136 to 162 megacycles for example. For this particular segment, all the housings and all the coils 25 may be identical. The resonant frequency desired for a particular helical resonator may then be attained according to the thickness, length, width, and/or direction of the tab 27 or 31. The dimensions and direction of tabs in the various resonators produced may be selected to divide the frequency segment of the range into convenient bands. The final fine adjustments within a particular band may be made by moving the annular member 29 until the preciseresonant frequency is reached.

Referring now to FIG. 4, a further embodiment of the invention is shown. The coil 41 might be utilized for operation at higher frequencies, such as, for example, 406 to 470 megacycles operation. Because of the higher frequencies used, fewer turns are required than were required in the coils of FIGS. 2 and 3. Support for the coil 41 is provided at the ends of the core member 43, and the portion 45 of the core member 43 intermediate its ends is reduced in diameter. This makes the coil 41 more efficient since it more closely approaches the ideal of an air core. As may be seen from this figure, the fiat tab 47 on the end of the coil 41 is supported on the enlarged end of g the core 43, and this tab may be of different sizes and shapes, and positioned in any of the ways described in connection with the coils of FIGS. 2 and 3.

It may therefore be seen that the invention comprises an improved construction for a helical resonator which permits mass production while providing for numerous variations of resonant frequency. Adjustment of such helical resonators may be made easily, and the construction is simple and efiicient.

We claim:

1. A helical resonator for use in a circuit responsive to applied signals of high frequency and having a housing forming a conductive cavity, and including in combination, an elongated fixed support member having a first end fixed to a wall of the cavity and a second end extending in the cavity, an inductive coil having first and second ends respectively supported by said tfirst and second ends of said support member, said support member providing mechanical rigidity for said coil, an elongated strip-like inductor portion supported by said second end of said support member and electrically connected to said second end of said coil, said inductor portion being shaped to conform to said second end of said support member, said inductor portion being dimensioned and orientated on said support member to provide a predetermined overall inductance of the helical resonator, and an annular member disposed in the cavity and having a surface surrounding said second end of said support member adjacent said inductor portion to form a capacitor with said inductor portion, with said dimensions of said inductor portion controlling the range of variability of the capacitance thereof, said annular member being linearly adjustable in the direction of the axis of said support member to vary the capacitance of said capacitor and having a split therein to prevent conduction therein about a closed loop, whereby said inductor portion functions as part of the inductance and as part of the capacitance of the helical resonator.

2. A helical resonator for use in a circuit responsive to applied signalsof a high frequency and having a housing forming a conductive cavity, said resonator including in combination, an elongated support member having a first end fixed to a wall of the cavity and a second end freely extending in the cavity, an inductive coil having first and second ends respectively supported by said first and second ends of saidsupport member, said support member providing mechanical rigidity for said coil, said support member being of substantially less diameter intermediate said first and second ends thereof, an elongated strip-like inductor portion supported by said second end of said support member and electrically connected to said second end of said coil, said inductor portion being shaped to conform to said second end of said support member, said inductor portion being dimensioned and being oriented upon said support member to provide a predetermined overall inductance of the helical resonator, and an annular member disposed in the cavity and having a surface surrounding said second end of said support member adjacent said inductor portion to form a capacitor with said inductor portion, said annular member being linearly adjustable in the direction of the axis of said support member to vary the capacitance of said capacitor, and having a split therein to prevent conduction therein about a closed loop, whereby said inductor portion functions as part of the inductance and as part of the capacitance of the helical resonator.

3. A helical resonator for use in a circuit responsive to applied signals of high frequency and having a housing forming a conductive cavity, said resonator including in combination, an elongated fixed rigid ceramic core within the cavity having a first end secured to the housing and a second end freely extending in the cavity, an inductive coil having first and second ends respectively supported by said first and second ends of said core so that said core provides mechanical rigidity for said coil, an elongated conductor shaped to conform to said second end of said core and supported thereon, said conductor being electrically connected to said second end of said coil and being dimensioned and orientated with respect to said coil to provide a predetermined overall inductance for the helical resonator, an annular conductive member disposed in the cavity and surrounding said second end of said core adjacent said conductor to form a capacitor therewith, said annular member having a gap therein to prevent conduction therein about a closed loop, and means supporting said annular conductive member which is adjustable in the direction of the axis of said core to change the position of said annular member with respect to said conductor to thereby vary the capacitance of said capacitor, whereby said conductor functions as part of the inductance and as part of the capacitance of the helical resonator.

4. A helical resonator for use in a circuit responsive to applied signals of high frequency and having a housing forming a conductive cavity, said resonator including in combination, an elongated fixed core within the cavity having a first end secured to the housing and a second end freely extending in the cavity, an inductive coil having first and second ends respectively supported by said first and second ends of said core so that said core provides mechanical rigidity for said coil, an elongated conductor portion shaped to conform to said second end of said core and supported thereon, said conductor portion being electrically connected to said second end of said coil and being dimensioned and orientated with respect to said coil so that said coil and said conductor portion together provide a predetermined overall inductance for the helical resonator, and an annular conductive member disposed in the cavity and surrounding said second end of said core adjacent said conductor portion to form a capacitor therewith, said annular member having a gap therein to prevent conduction therein about .a closed loop and being adjustable in the direction of the axis of said core to vary the capacitance of said capacitor, whereby said conductor portion functions as part of the inductance and as part of the capacitance of the helical resonator.

References Cited by the Examiner UNITED STATES PATENTS Franklin et a1 334-80 X Ussel-man et a1. 317-249 Kihn 333-82 X Kiebert et al. 334-66 Wagner 317-249 Van Duyne 317-249 Horvath 333-77 X Dreyer 334-68 Ketchu-m 334-80 Czubiak et a1 333-83 HERMAN KARL SAALBACH, Primary Examiner. 

1. A HELICAL RESONATOR FOR USE IN A CIRCUIT RESPONSIVE TO APPLIED SIGNALS OF HIGH FREQUENCY AND HAVING A HOUSING FORMING A CONDUCTIVE CAVITY, AND INCLUDING IN COMBINATION, AN ELONGATED FIXED SUPPORT MEMBER HAVING A FIRST END FIXED TO A WALL OF THE CAVITY AND A SECOND END EXTENDING IN THE CAVITY, AN INDUCTIVE COIL HAVING FIRST AND SECOND ENDS RESPECTIVELY SUPPORTED BY SAID FIRST AND SECOND ENDS OF SAID SUPPORT MEMBER, SAID SUPPORT MEMBER PROVIDING MECHANICAL RIGIDITY FOR SAID COIL, AN ELONGATED STRIP-LIKE INDUCTOR PORTION SUPPORTED BY SAID SECOND END OF SAID SUPPORT MEMBER AND ELECTRICALLY CONNECTED TO SAID SECOND END OF SAID COIL, SAID INDUCTOR PORTION BEING SHAPED TO CONFORM TO SAID SECOND END OF SAID SUPPORT MEMBER, SAID INDUCTOR PORTION BEING DIMENSIONED AND ORIENTATED ON SAID SUPPORT MEMBER TO PROVIDE A PREDETERMINED OVERALL INDUCTANCE OF THE HELICAL RESONATOR, AND AN ANNULAR MEMBER DISPOSED IN THE CAVITY AND HAVING A SURFACE SURROUNDING SAID SECOND END OF SAID SUPPORT MEMBER ADJACENT SAID INDUCTOR PORTION TO FORM A CAPACITOR WITH SAID INDUCTOR PORTION, WITH SAID DIMENSIONS OF SAID INDUCTOR PORTION CONTROLLING THE RANGE OF VARIABILITY OF THE CAPACITANCE THEREOF, SAID ANNULAR MEMBER BEING LINEARLY ADJUSTABLE IN THE DIRECTION OF THE AXIS OF SAID SUPPORT MEMBER TO VARY THE CAPACITANCE OF SAID CAPACITOR AND HAVING A SPLIT THEREIN TO PREVENT CONDUCTION THEREIN ABOUT A CLOSED LOOP, WHEREBY SAID INDUCTOR PORTION FUNCTIONS AS PART OF THE INDUCTANCE AND AS PART OF THE CAPACITANCE OF THE HELICAL RESONATOR. 